The present invention relates to an antenna apparatus for making communication with a radio frequency IC tag (RF ID tag) to read information therein and serving as a reader/writer.
Recently, radio frequency IC tags are widely used for information management or distribution management of articles and structures. Such radio frequency (RF) IC tags each include a small IC chip in which information is recorded and a transmitting antenna for transmitting the information recorded in the IC chip by radio. The small IC chip having, for example, a thickness of 0.4 mm, a depth of 0.4 mm and a height of 0.1 mm is utilized. A reader/writer is merely held to the radio frequency IC tag to thereby make it possible to get information concerning attributes of individual articles or structures without contact. It is desirable that the radio frequency IC tag is as small as possible in order to stick the radio frequency IC tag on an article or embed the radio frequency IC tag in a structure. For example, when the radio frequency IC tag is stuck on the head of a bolt, the radio frequency IC tag is desirably smaller than or equal to 10 mm in size.
In order to make the radio frequency IC tag small, it is necessary to make small a transmitting antenna for transmitting information. In this connection, for example, when the length of the transmitting antenna of the radio frequency IC tag is made smaller than or equal to λ/4 where λ is a wavelength of radio wave transmitted by the radio frequency IC tag, a general reader antenna such as a dipole antenna and a patch antenna used generally has insufficient sensitivity due to reduction of antenna efficiency and it is necessary to use a reader antenna of a special shape (special reader antenna).
FIG. 16 shows an image of an antenna of a radio frequency IC tag that is cut for use.
A standard inlet 51 (1.5 mm wide and 51 mm long) having the antenna length of λ/4 is cut at both ends thereof to form a small inlet 52 (1.5 mm wide and 10 mm long) having the antenna length of λ/10, for example. When this small inlet is used as a small radio frequency IC tag 53, communication is made using a special reader antenna 54 as shown in FIG. 16 due to reduction of antenna efficiency. In this case, it is necessary to exactly align the radio frequency IC tag 53 with the special reader antenna 54 connected through a cable 55 to a reader/writer 56 and it is not easy to handle. Further, such a special reader antenna is usually designed in accordance with the size and the shape of the radio frequency IC tag and accordingly it has no generalization.
Heretofore, there is known, for example, a special reader antenna including a resonator composed of a capacitor and a coil disposed at the tip of a pen-type reader (for example, JP-A-2002-183676 (paragraphs 0005-0008)).